This invention relates generally to tuning resistors to produce more accurate resistors.
Termination impedances are often used at the transmit and receiving ends of devices passing high speed electrical signals over a transmission line media to reduce reflections and the subsequent signal degradation caused by those reflections. Reflections may degrade the signals at both the transmitter and receiver. The closer the termination impedance matches the impedance of the transmission line, the smaller the reflections. The reflections increase as the speed of the signals gets higher relative to the round trip delay of the media.
Termination resistors are sometimes implemented on integrated circuits to minimize these reflections when high speed signals are being transmitted or received. Accurately matching the termination resistors to the value of the impedance of the attached transmission line reduces reflections and resultant signal degradation. Thus, it is desirable that the termination resistor be very accurate across a range of temperatures and power supply voltages.
Termination resistors may be implemented in commonly available integrated circuit technologies materials such as in a diffusion, polysilicon, or metal. The resulting resistors are not very accurate and vary substantially with temperature and power supply voltage. This variation in resistance may result in a significant mismatch between the on-chip termination resistor and off chip transmission line, and hence significant reflections may result. This level of signal degradation may not be acceptable in some applications.
One method used to make more accurate termination resistors on an integrated circuit is to use an integrated circuit process that makes very accurate resistive elements. However, making accurate resistor elements using integrated circuit process techniques substantially increases the cost of the process.
A second method to implement more accurate termination resistors on an integrated circuit is to trim an inaccurate resistor to an accurate value. This can be done by fuse trimming or laser trimming the resistor to the desired value at manufacturing. The drawback of this approach is that it consumes silicon area and only adjusts absolute value. This approach is not able to compensate for resistor variations with temperature and power supply voltage.
Thus, there is a need for better ways to improve the accuracy of resistors.